Loose objects such as loose metal objects, e.g. bolts, screws, plates and others, can cause serious damage inside a wind turbine, in particular to those parts which are being moved during the operation of the wind turbine and/or to electric, electronic respectively hydraulic functional parts. Such loose object will be transported by a moving part, in particular by a rotating part such as the hub of the wind turbine. The loose objects inside a hub are lifted up to some height and will then fall down again. For instance, if a loose object lies on the bottom ground of a hub as the hub starts to rotate, the object will be transported higher due to the rotation movement of the hub and will the fall down again due to gravity. This goes on repeatedly during operation of the wind turbine so that more and more damage is caused, in particular in those regions in which objects can hit particularly vulnerable parts.
A solution of monitoring loose objects inside wind turbines has been proposed by U.S. Pat. No. 6,785,637 B1, namely acoustic monitoring. The method outlined there includes that an actually acquired frequency spectrum of sounds of the wind turbine is substracted from a reference frequency spectrum. This resulting difference spectrum may then be further inspected and an alarm may be set off in case of abnormalities. In particular, the difference spectrum can be monitored by staff in order to find out about such abnormalities. It is clear that such monitoring system and method is rather complex and requires large resources of computational power and in some cases also of manpower. Furthermore, the methodology of evaluation of abnormalities is rather vague as it is at least partially based on human judgement rather than on objective criteria.